Method for Producing Steel Pipe Plated With Metal by Thermal Spraying

ABSTRACT

[Problems] To provide a process for producing a thermal sprayed, metal plated steel tube having a metal layer substantially uniform and favorable over the whole surface in a highly productive manner.  
     [Means for solving] A process for producing a metal tube comprising continuously forming a metal plate composed of a first component into a tubular shape and continuously welding its butted ends to form the metal tube, wherein, after the continuous welding, a metal layer composed of a second component which is different from the first component is formed by thermal spraying directly over the surface of the metal tube, the metal layer having no discontinuities in the circumferential and lengthwise directions. Especially it is preferred that the process comprises a process step of making the thickness distribution of the metal layer more uniform after formation of the thermal sprayed metal layer over the external surface.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a metal platedsteel tube, comprising a thermal spraying step to the internal orexternal surface of the steel tube.

Traditionally, processes for producing metal plated steel tubes areknown which comprise a step of continuously forming a steel plate thatis plated on both sides with a heterogeneous metal (typically, zinc)into a tubular shape using roll forming, a step of continuously weldingthe tubular plated steel plate so formed at its end faces to form atube, a step of continuously cutting weld beads that are formed on theexternal surface of the steel tube during the previous step and a stepof thermal spraying portions where part of the plated layer has beenremoved due to the cutting with the same metal as the one for theplating or with a heterogeneous metal (Patent Reference 1).

In such steps, it is also contemplated that the final thermal spraycoating of the unplated portions may be substituted with continuousmolten plating (Japanese Patent Application No. 2002-225668 by theApplicant (unpublished)). Patent Reference 1: Japanese Unexamined PatentPublication No. 1993-148607.

In such a field of art, thermal spraying is exclusively used for repairapplications. It means that the idea of thermal spraying metal tubespartially with a metal on their unplated portions in order to shieldsuch portions and the like from the ambient atmosphere exists. Such anidea that, in order to modify a whole plated layer, thermal spraying awhole tube instead of specific portions thereof to form a metal layerall over the tube does not, however, exist.

On the other hand, for the purpose of modifying a whole plated layerinstead of repairing, steps are known in which molten plating is alsoapplied using a similar or dissimilar metal to the steel tube producedaccording to the steps for production described above. For example, itis known that by incorporating aluminum to zinc plating, corrosionresistance that may not be obtained with zinc alone may be realized. Asenvironments where products are used become increasingly stringent yearafter year, with lengthening of high corrosion resistance and servicelife being desired, there is a need for new alloy plating. As such,characteristics required include wear resistance, heat resistance,electrical insulation, electromagnetic shielding and electricalconductivity, in addition to corrosion resistance, all of which may berealized by coating with other metallic and nonmetallic materials in acomposite manner.

Conventionally, the step of coating with multiple heterogeneous metalsinvolves forming a steel plate plated with zinc on both sides using rollforming and the like, followed by continuous welding and removal ofbeads and the like, before molten plating with zinc or a different metal(aluminum, for example). However, providing anew a molten plating bathalong an existing line will present a difficulty in terms of processdesign and arrangement and initial investment will be substantial.

SUMMARY OF THE INVENTION

Metal plated steel tubes as produced by processes using continuousthermal spraying suffer from disadvantages as follows. Since bondingstrength of thermal sprayed metal layers at the interface with the steeltubes is poorer in comparison with that of molten plated layers, when aprocessing such as bending is applied, the thermal sprayed metal layerswill develop delamination and/or cracking, potentially causing problemsin corrosion resistance of the steel tubes. Also, it is difficult tofinely control the thickness of the thermal sprayed metal layers, makingit difficult to form thermal sprayed metal layers in a uniform manner.

Proposed in the light of the above, the present invention has an objectof providing a process for producing a thermal sprayed, metal platedsteel tube having a substantially uniform and favorable metal layer overthe whole surface in a highly productive manner. The present inventionhas another object of forming a metal layer having sufficient bondingstrength with a metal layer of a plated layer.

In order to attain the objects described, a process for producing athermal sprayed metal plating according to the present inventioncomprises a step of continuously thermal spraying a steel plate, a stepof continuously forming the steel plate, a step of continuously weldingthe steel plate formed into a tubular shape at the joint of lengthwiseend faces to form a steel tube, a step of continuously molten platingthe external surface of the steel tube and a step of continuouslythermal spraying the external surface of the steel tube.

The present invention may be represented in a more functional manner asfollows. Specifically, the present invention relates to a process forproducing a metal tube comprising continuously forming a metal platecomposed of a first component into a tubular shape and continuouslywelding its butted ends to form the metal tube, wherein, prior to thecontinuous welding, a metal layer composed of a second component whichis different from the first component is formed by thermal spraying overat least one surface of the metal plate, the metal layer having nodiscontinuities in the circumferential and lengthwise directions.

Also, the present invention relates to the process for producing a metaltube, wherein, after the continuous welding, the metal layer composed ofa second component which is different from the first component is formedby thermal spraying directly over the surface of the metal tube, themetal layer having no discontinuities in the circumferential andlengthwise directions.

Also, the present invention relates to the process for producing a metaltube, wherein, after the continuous welding, the metal layer having nodiscontinuities in the circumferential and lengthwise directions isformed by thermal spraying through a metal layer composed of a componentwhich is different from the first component over the surface of themetal tube.

According to the present invention, formation of the metal layer bythermal spraying is performed preferably to the internal surface priorto the continuous welding and to the surface of the tube after thecontinuous welding and may also be performed in combination.

According to the process of the present invention, a metal tubecomprising a first metal tube portion composed of a first component, thefirst metal tube portion having a lengthwise continuous molten bondedsection on at least part of the cross section of the first metal tubeportion, and a second metal layer formed directly or through a metallayer composed of a component which is different from the firstcomponent over any of the surfaces of the first metal tube portion, thesecond metal layer being formed by thermal spraying continuously in thedirection of the cross section can be produced.

Further, the present invention may be characterized that sufficientbonding strength with the metal layer composing the plated layer isprovided by performing thermal spraying when the surface of the platedlayer is not completely cooled or, preferably, is semi-molten. The term“semi-molten” means a condition midway toward coagulation at atemperature from the melting point to about 80% of the melting point,for example, from the melting point to 400° C. in case of zinc as aplated layer.

The plated layer for the metal tube obtained by the present invention ischaracterized that the metal having a second component is scattered inthe shape of islands in the metal having a first component and that themetal having the second component is distributed in a layered manner inthe metal having the first component.

A metal to be used for thermal spraying and molten metal plating maypreferably be zinc, but it may also be an alloy containing aluminum orother metals in addition to zinc or may be another metal such as tin.

The term “thermal spraying” as used herein is in accordance with generaldefinitions based on well-known metallurgical technology. As one of suchdefinitions, the term refers to a process for heating a material to bethermal sprayed using combustion or electrical energy and blowingparticles rendered molten or substantially molten to a substrate to forma coated film (refer to JIS Handbook: Metal Surface Treatment H8200).

According to the present invention, a metal layer is formed over theinternal or external surface of a metal tube by thermal spraying,instead of a conventional plating step, such as molten plating, with aresult that highly expensive installation and maintenance of platedlayers may be dispensed with, a wide variety of metal layers may beformed with low-cost initial investment and the degree of freedom fordesigning metal tubes may be increased.

According to the present invention, in contrast to conventional localrestoration of a metal tube by thermal spraying, a metal layer that iscontinuous in the cross sectional or circumferential direction, such asthe whole surface of a metal tube, is formed by thermal spraying. Use ofthermal spraying that is not intended for local restoration is notfamiliar with those skilled in the art.

According to the present invention, a thermal sprayed, metal platedsteel tube that is excellent in corrosion resistance and effective inrust prevention may be obtained by forming an aluminum layer and analloy metal containing aluminum in zinc plating using a thermal sprayingdevice.

According to the present invention, positional relationship between asubstrate metal and a thermal sprayed metal (distribution relationship)may be controlled when a plated layer is composed of an alloy containingmultiple metals or compositions.

The effects and other features of the present invention as describedherein shall not be construed as limiting the scope of rights of thepresent invention. It is needless to say that embodiments comprisingsome, but not all, of such effects and features may also fall within thescope of rights of the present invention on the basis of interpretationof rights established in national laws and practices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a production line according to oneembodiment of the present invention;

FIG. 2 is a longitudinal section of thermal sprayed metal platingproduced by a process according to the embodiment;

FIG. 3 shows a result of surface analysis of Al element over asurface-treated layer of a metal tube produced in this example;

FIG. 4 shows a result of surface analysis of Al element over asurface-treated layer of a metal tube produced in this example;

FIG. 5 shows external appearances of metal tubes produced in Example 1;

FIG. 6 shows results of line analysis of a surface-treated layer of ametal tube produced in Example 2; and

FIG. 7 shows results of line analysis of a surface-treated layer of ametal tube produced in Example 3.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below indetail.

A process for producing a thermal sprayed, metal plated steel tubeaccording to the embodiment comprises a production line as shown inFIG. 1. The production line includes, at least, an uncoiler 2 forfeeding a continuous steel plate wound around a coil 1, a forming device5 for continuously forming the steel plate fed from the uncoiler 2 intoa tubular shape, an internal surface thermal spraying device 4 forthermal spraying the steel plate with a desired metal immediately beforecontinuously forming the steel plate into a tubular shape, a weldingdevice 7 for welding the joint of lengthwise end faces of the platedsteel plate formed into a tubular shape to form a tubular body, acutting device 8 for continuously cutting weld beads formed on theexternal surface of the tubular body, and a molten zinc plating device11 for continuously molten zinc plating the external surface of thetubular body to form a molten zinc plated steel tube.

It is also possible to eliminate the internal surface thermal sprayingdevice 4 from the above arrangement and to provide instead an externalsurface thermal spraying device 12 for thermal spraying, subsequent tothe molten zinc plating device 11, if necessary.

It is also possible to provide the internal surface thermal sprayingdevice 4 in conjunction with the external surface thermal sprayingdevice 12 along the production process. Such arrangement depends on thespecifications of steel tubes to be produced.

If necessary, a flux application device 9 for continuously applying aliquid flux for cleansing and antioxidizing the external surface of thetubular body and a preheating device 10 for simultaneously drying theexternal surface of the tubular body and preheating the tubular body mayalso be provided. Also, a sizing device 13 for shaping the molten zincplated steel tube thus produced into specified dimensions and asectioning device 14 for sectioning the molten zinc plated steel tube ina predetermined length may be provided.

A process according to the present invention using the above productionline will then be described.

First, a steel plate wound as a coil is continuously fed from theuncoiler 2 downstream along the line.

The steel plate fed is introduced into the forming device 5, beingsimultaneously thermal sprayed by the thermal spraying device 4, to becold formed to be tubular. The thermal spraying should preferably beapplied to one of the surfaces of the steel plate and the thermalsprayed surface should preferably be the internal surface of the formedtube. The thermal spraying may be performed before or after the start offorming. Subsequently, the joint of lengthwise end faces is continuouslywelded by the welding device 7 to form a single continuous tubular body6.

The tubular body 6 is then fed to the cutting device 8 comprising anattached blade conformal to the contour of the tubular body 6. Weldbeads formed on the external surface of the tubular body 6 are thenscraped off by the blade of the cutting device 8 to smooth the externalsurface of the tubular body 6.

The tubular body is subsequently fed to the flux application device 9 tobe applied with a liquid flux for cleansing and antioxidizing theexternal surface of the tubular body.

The tubular body 6 is fed to the preheating device 10 to be preheatedwhile the external surface is dried.

Thereafter, the tubular body is fed to the molten zinc plating device11, where the tubular body 6 is immersed in an upper plating bath thatis filled with pumped-up molten zinc so that the whole external surfacemay be plated with the molten zinc. The tubular body 6 that has beenimmersed in the upper plating bath is provided with a molten zinc platedlayer having a wholesome alloy layer and is now a molten zinc platedsteel tube. Then, excess molten zinc plating is removed at a wipingdevice (not shown) and, then, a thermal sprayed, metal plated steel tube40 is completed by the external surface thermal spraying device 12.Cooling is then carried out.

During this time, the thermal spraying by the external surface thermalspraying device 12 needs to be carried out before the surfacetemperature of the molten zinc plated layer drops to the ambienttemperature. In general, a thermal sprayed metal layer formed over acooled metal surface is uniformly formed along irregularities on themetal surface and thereby secures bonding strength only through theanchor effect by such irregularities. When thermal spraying is carriedout in such conditions that the surface temperature of a molten zincplated layer may not drop to the ambient temperature as in the presentinvention, however, part of the thermal sprayed metal may infiltrateinto the molten zinc plated layer, or an alloy layer or an elementdiffusion layer may be formed between the molten zinc plated layer andthe thermal sprayed layer, so that the bonding strength may be enhancedby other factors than the anchor effect.

According to the present invention, it is preferable to carry outthermal spraying in such conditions that the surface temperature of amolten zinc plated layer is higher than the ambient temperature. It ismore preferable that the surface of a molten zinc plated layer issemi-molten. A semi-molten refers to a condition at temperatures rangingfrom a temperature of solid-liquid coexisting region to several tens of° C. or, at most, 100° C. lower than the temperature of solid-liquidcoexisting zone, when a metal layer of a plated layer is of a metal witha composition having such a solid-liquid coexisting region. When a puremetal composes such a plated layer, there is no such temperature ofsolid-liquid coexisting region. In this case too, however, substantiallythe same will hold true. Temperatures of a semi-molten conditionpreferable for the present invention may vary depending on thecombination of a plated layer and a thermal sprayed metal layer. Ingeneral, any high temperatures capable of producing bonding strengththrough element diffusion and/or alloying between a plated layer and athermal sprayed layer or other mechanisms than the anchor effect areapplicable for the purpose of the present invention.

According to this embodiment, a metal layer by thermal spraying must beformed circumferentially all over the surface of a tube. As such,according to this embodiment, the external surface thermal sprayingdevice is provided with three thermal spray nozzles 120° apart indirection from each other to carry out thermal spraying of a metal. Athermal sprayed metal layer can be formed with thermal spraying from twodirections 180° apart from each other; however, thermal spray nozzlesshould desirably be provided in three or more directions to form athermal sprayed metal layer.

The thermal sprayed, metal plated steel tube 40 is then cold rolled atthe sizing device 13 in order to produce the external shape complyingwith specified dimensions. Cold rolling is a step necessary in thisembodiment to form the thermal sprayed layer with a relatively eventhickness in the circumferential direction. In other words, even whenthe thermal sprayed metal layer has unequal thickness in thecircumferential direction immediately after being formed by the externalsurface thermal spraying device, subsequent cold rolling and the likecan even out the thermal sprayed metal layer with a relatively uniformthickness. Thus, according to the preferred embodiment of the presentinvention, a step of sizing, such as cold rolling, a thermal sprayedmetal layer after the formation of the thermal sprayed metal layer bythe external surface thermal spraying device to provide the thermalsprayed metal layer with a relatively uniform thickness (a step ofmaking the thickness distribution more uniform than that immediatelyafter the formation of the thermal sprayed metal layer) may desirably beadopted.

The thermal sprayed, metal plated steel tube is sectioned by thesectioning device 14 in a predetermined length to be a steel tubeproduct 15.

According to the process for producing a thermal sprayed, metal platedsteel tube as designed above, a thermal sprayed metal plated steel tubethat is excellent in corrosion resistance and effective in rustprevention may continuously be obtained by providing the tubular body 6having thereon a molten zinc plated layer with a thermal sprayed metallayer M over the internal surface and a thermal sprayed metal layer Oover the external surface. In FIG. 2, the thermal sprayed metal layer Ois formed externally around a zinc plated layer N.

The present invention is not limited to the above embodiment. Forexample, in the above embodiment, thermal sprayed metal layers areformed by a thermal spraying device over both the external and internalsurfaces; however, a thermal sprayed metal layer may be provided by athermal spraying device only over either one of the external andinternal surfaces. In addition, when a thermal sprayed metal layer isprovided only over the external surface, use of a double-side platedsteel plate can produce a steel tube provided with a plated layer overthe internal surface and a double metal layer (plated layer plus thermalsprayed metal layer) over the external surface. Furthermore, when ametal plated steel plate that has a plated layer over one side is used,a thermal sprayed metal layer may be formed over the other surface withno metal plating so that both the internal and external surfaces of atube may have a metal layer to enhance the anti-corrosive effect.

Also, the top surface of the thermal sprayed metal layer may be coatedwith a protective film of a synthetic resin and the like. Thus, the rustpreventive effect of the thermal sprayed, metal plated tube may furtherbe enhanced.

Further in this embodiment, molten zinc is utilized as a plating to beapplied to a steel tube; however, if necessary, other metals may also beutilized. Also in this embodiment, description was made with anassumption that a steel plate is used; however, the present inventionmay be premised on the use of other metal plates. Such metal platesinclude, but not limited to, copper tapes, aluminum tapes and the like.

The surface treatment process according to the present invention isapplicable not only to metal tubes but also to any metal members. Suchmetal members would have metal surfaces produced by plating a metalsurface with a molten metal having a first component to produce a firstmetal surface and thermal spraying the surface of the first metal layerwith a metal having a second component to provide a surface layer inwhich the metals having the first and second components are blendedtogether.

EXAMPLES

Working examples of processes for producing thermal sprayed, metalplated steel tubes will be illustrated below.

Example 1

Aluminum was used as a metal to be thermal sprayed. A rolled continuoussteel plate with a thickness of 1.2 mm and a width of 59.5 mm was set ina production line as shown in FIG. 1 and was processed with shotblasting on the internal surface with a shotblast, followed by beingapplied with a thermal sprayed metal layer over the internal surfacewith a thermal spraying device. In addition, after a molten zinc platingdevice, aluminum was thermal sprayed with an external surface thermalspraying device.

According to the present invention, conditions such as temperatures ofthe surface of zinc at the time of thermal spraying (ordinarytemperature to 450° C.), line speeds (0 to 400 m/min) and sprayingangles (0 to 90°) can be combined as appropriate. Concurrently, thebonding concentrations and distribution of aluminum can be controlled byadjustment of the amounts of thermal sprayed aluminum.

FIG. 5 shows in comparison appearances of a steel tube produced byExample 1 and a steel tube produced by a molten plating process as aprior art. According to the molten plating process as a prior art, thesurface exhibits a pattern (spangle) reflecting its unique grainboundaries (FIG. 5 (a)) while the surface of the steel tube produced bythe thermal spraying of Example 1 is coarsely granular (FIG. 5 (b)).Plated steel tubes exhibiting such coarse granular surfaces were notknown at the time of filing of the present application and, therefore,it is assumed that the plated steel tube having such a surface has beenproduced by the process according to the present invention. In addition,there is shown an appearance of a steel tube produced by Example 1 whichwas polished with a sandpaper (FIG. 5 (c)). The sandpapered steel tubeis metallically lustrous with punctiform recesses or pits that areunique to thermal spraying and, therefore, it is assumed that the tubehaving such an appearance has also been produced by the processaccording to the present invention.

FIG. 3 schematically shows the result of Al elemental analysis for theproduct produced in Example 1. As a result of elemental analysis on thethermal sprayed metal layer over the external surface, it is observedthat the thermal sprayed metal (aluminum) is scattered in the shape ofislands throughout the zinc layer, 20. Also, FIG. 4 shows the result ofAl elemental analysis when thermal spraying was carried out underanother set of conditions. Contrary to FIG. 3, Al is segregated in thevicinity of the steel wire (St) and distributed in a layered manner inthe vicinity of the steel wire in a location that is different from Znand is farther as seen along the direction of thermal spraying. Thus,the plated layer produced by the process according to the presentinvention is characterized for one thing by that a thermal sprayed metalis scattered in the shape of islands and/or is distributed in a layeredmanner in a substrate metal. In addition, in Example 1, it is apparentthat the plated layer and the thermal sprayed layer are not bonded onlythrough the anchor effect.

Such distribution in the shape of islands or in a layered manner is oneof the characteristics when the process according to the presentinvention is used; however, such a characteristic may disappear whenheat treatment or the like is applied after the process according to thepresent invention was applied. It may be assumed that the plated layerhaving such a characteristic was produced by the process according tothe present invention.

Inferring details of the principle for such distribution, the processaccording to the present invention adopts thermal spraying in which ametal that is different from a plated substrate metal is sprayed throughphysical force, in contrast to conventional molten plating in multipletimes and, therefore, distribution between the plated substrate metaland the thermal sprayed metal is influenced at least by the temperatureof the plated substrate metal at the time of thermal spraying (whichinfluences the hardness of the surface and the like), the temperature ofthe thermal sprayed metal at the time of thermal spraying (whichinfluences the hardness of the thermal sprayed metal) and the rate atwhich the thermal sprayed metal reaches the plated substrate metal(kinetic energy). Based on these factors, distribution as shown in FIG.3 is formed under the conditions where the thermal sprayed metalconvects in the relative vicinity of the surface. On the other hand,distribution as shown in FIG. 4 may be formed under the conditions wherethe thermal sprayed metal reaches the vicinity of the surface of thesteel wire (St).

Example 2

Aluminum was used as a metal to be thermal sprayed, and a rolledcontinuous steel plate with a thickness of 1.2 mm and a width of 59.5 mmwas set in a production line as shown in FIG. 1 and was processed withshot blasting on the internal surface with a shotblast device, followedby being applied with a thermal sprayed metal layer over the internalsurface with a thermal spraying device. In addition, after molten zincplating, aluminum was thermal sprayed with an external surface thermalspraying device.

A photograph of the appearance of the obtained section is shown in FIG.6 (a) and the elemental analysis of the section as determined by EPMA isshown in the chart of FIG. 6 (b). As shown in FIG. 6 (a), characteristicscattering of the thermal sprayed metal in the shape of tiny islandsthroughout the substrate metal is observed also in this example. It isseen that tiny dots of blackish aluminum are scattered throughout thewhitish zinc substrate. In addition, as shown in FIG. 6 (b), the zincand aluminum are distributed in relatively uniform concentrations in thesteel tube produced in this example.

Example 3

Aluminum was used as a metal to be thermal sprayed, and a rolledcontinuous steel plate with a thickness of 1.2 mm and a width of 59.5 mmwas set in a production line as shown in FIG. 1 and was processed withshot blasting on the internal surface with a shotblast, followed bybeing applied with a thermal sprayed metal layer over the internalsurface with a thermal spraying device. In addition, after a molten zincplating device, aluminum was thermal sprayed with an external surfacethermal spraying device. In this example, aluminum was thermal sprayedat 15 g/min with a surface temperature of the zinc plating of 400° C.during a low production rate of 20 m/min with a spraying angle of 90°.Contact between the zinc and the thermal sprayed aluminum fuses topromote bonding with the aluminum so that distribution of an aluminumlayer at the surface, a zinc-aluminum layer in-between and a zinc layerat the innermost layer may be obtained. This arrangement is feasible ata medium to high production rate depending on a series ofcorrelationship between line speeds, surface temperatures of zincplating, amounts of sprayed aluminum, and the like.

The obtained section is shown in FIG. 7 (a) and the elemental analysisof the section as determined by EPMA is shown in the chart of FIG. 7(b). As shown in FIG. 7 (a), according to the present invention,aluminum (blackish part) is eccentrically located at the surface andzinc (whitish part) is located at the interface with the steel core.Examined more closely by elemental analysis, the distribution exhibitsan substantially pure aluminum layer formed at the surface and a gradualreversal in the concentrations of the aluminum and zinc, as shown inFIG. 7. Reasons for such element distribution are not clear; however, itmay presumably be due to the change in depth of infiltration of thealuminum into the zinc as a thermal sprayed metal layer, depending onthe fusing temperature of the aluminum at the time of thermal spraying,the temperature of the zinc as a thermal sprayed metal layer and theinitial velocity (kinetic energy) of the thermal sprayed metalparticles. This suggests the possibility of arbitrarily controlling theelement distribution in a metal layer by adjustment in such parameters.

The product of this embodiment is composed of an aluminum layer with apurity of almost 100% down to a depth of 50 μm from the surface and analloy layer of aluminum and zinc in the deeper section. In general,since pure aluminum is highly corrosion resistant, a product with suchelement distribution is more corrosion resistant as a plated steel tube.The increased corrosion resistance is partly attributable to that evenif minute defects (pinholes) exist in the aluminum layer, the zinc layerwill provide a sacrificial corrosion preventive action, bearing ananti-corrosive function through a mechanism of action that is differentfrom that of aluminum. According to the present invention, a producthaving a pure aluminum layer at the surface and an alloy layer ofaluminum and zinc or a zinc layer in the interior may be obtained. Insuch cases, the pure aluminum layer has a thickness of 30% or more,preferably 50% or more, of the thickness of the whole plated layer. Thealuminum layer at the surface does not necessarily have to be analuminum layer with a purity of 100% as long as it has corrosionresistance that is substantially equal to that of 100% aluminum. Fromthis point of view, inclusion of other elements such as zinc in theorder of 1 to 5% is tolerable.

This shows that it is possible to control not only the composition butalso the metal distribution of a plated layer by adopting the processaccording to the present invention.

Although description was made herein by way of example using zinc as ametal to be used for molten metal plating, it is not limited thereto.For example, an alloy containing aluminum or other metals in addition tozinc or another metal such as tin may also be used. In addition,although description was made herein by way of example using aluminum asa metal to be thermal sprayed, it is not limited thereto. For example,zinc, magnesium or other metals may also be used. The present inventioncomprises an invention relating to a process for production and, atleast in an invention relating to a process for production, the identityof metals or alloys may not matter.

1. A metal tube comprising a first metal tube portion composed of afirst component, the first metal tube portion having a lengthwisecontinuous molten bonded section on at least part of the cross sectionof the first metal tube portion, and a second metal layer formed overany of the surfaces of the first metal tube portion through a metalplated layer composed of a component which is different from the firstcomponent, the second metal layer being formed by thermal sprayingcontinuously in the circumferential and lengthwise directions.
 2. Aprocess for producing a metal tube comprising continuously forming ametal plate composed of a first component into a tubular shape andcontinuously welding its butted ends to form the metal tube, wherein,prior to the continuous welding, a metal layer composed of a secondcomponent is formed by thermal spraying over at least one surface of themetal plate across the whole width of the metal plate through a metalplated layer composed of a component which is different from the firstcomponent.
 3. (canceled)
 4. A process for producing a metal tubecomprising continuously forming a metal plate composed of a firstcomponent into a tubular shape and continuously welding its butted endsto form the metal tube, wherein, after the continuous welding, a metallayer having no discontinuities in the circumferential and lengthwisedirections is formed by thermal spraying over a surface of the metaltube through a metal layer composed of a component which is differentfrom the first component.
 5. The process according to claim 4, furthercomprising a process step of making the thickness distribution of themetal layer more uniform after formation of the metal layer having nodiscontinuities in the circumferential and lengthwise directions.
 6. Ametal tube comprising a first metal layer having a first component,formed by metal plating, over a surface of the metal tube directly orthrough a metal layer composed of a component which is different fromthe component of the metal tube and a second metal portion having asecond component, formed by thermal spraying, over the surface of thefirst metal layer, wherein the second metal portion is scattered in theshape of islands in the first metal layer.
 7. A metal tube comprising afirst metal layer having a first component, formed by metal plating,over a surface of the metal tube directly or through a metal layercomposed of a component which is different from the component of themetal tube and a second metal portion having a second component, formedby thermal spraying, over the surface of the first metal layer, whereinthe second metal portion is distributed in a layered manner in the firstmetal layer.
 8. A metal tube comprising a zinc layer formed by moltenmetal plating over a surface of the metal tube directly or through ametal layer composed of a component which is different from thecomponent of the metal tube and an aluminum layer formed by thermalspraying over the surface of the zinc layer, wherein the aluminum layerhas a thickness of 30% or more of the thickness of the total thicknessof the zinc and aluminum layers.
 9. The metal tube according to claim 7or 8, wherein the surface is coarsely granular or has metallic lusterand punctiform pits.